Polycrystalline, randomly oriented oxide superconductors exhibit critical current densities orders of magnitude lower than that of single crystal or highly oriented materials. The decrease in critical current density, as measured across grain boundaries in a randomly oriented material, is attributed to grain boundary impurities or grain boundary mismatch. The decrease in critical current density (J.sub.c) as measured across grain boundaries and, in particular, in the presence of a magnetic field is known as weakly-linked behavior.
Grain alignment or crystallographic texture has been shown to mitigate weakly-linked behavior of certain oxide superconductors and to permit "strongly-linked" behavior, allowing high current densities even in strong magnetic fields. Techniques used to produce grain alignment in oxide superconductors include melt texturing, magnetic alignment and mechanical deformation. Both melt texturing and magnetic alignment suffer from extremely slow processing speeds and the inability to process large amounts of material.
Melt texturing has been used to process oxide superconductors, in particular, the yttrium barium cuprate (YBCO) and bismuth strontium calcium cuprate (BSCCO) systems. Melt textured oxide superconductors have good grain alignment, high critical current density and strongly-linked grains. These qualities are obtained by forming large (millimeter scale) well-aligned grains. While such large grains are highly desirable for current transport, melt textured materials are brittle and can not be easily formed into complex shapes. A fine grained material (micron scale) would have the greater flexibility required for bulk applications such as current-carrying wires.
Mechanical deformation is widely used in the metal and ceramic arts to induce alignment by a purely mechanical means, for example, the alignment of fibers in a fiber-reinforced metal. In ceramic and ceramic composite materials, deformation processing is used to rotate, and thereby align, grains and particles having high aspect ratios. Grain rotation by mechanical means in brittle materials, such as oxide superconductors, is challenging because of the tendency for the oxide superconducting grains to fracture and pulverize upon deformation.
It is generally felt in the art that RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 (124) type oxide superconductors have inherently poor grain boundaries and are weakly-linked, where RE designates rare earth elements. By analogy, the RE.sub.2 Ba.sub.4 Cu.sub.7 O.sub.x (247) type oxide superconductors which are structurally related to the 124-type compound might be also considered weakly-linked. For example, Shibutani et al investigated the critical current characteristics of wires prepared from [Y(Ca)].sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 by mechanical deformation and reported the wires to have poor critical current retention (&lt;20% at approx. 0.1T). Shibutani et al. characterized the material as having incomplete grain boundary coupling and as being weakly-linked.
It is therefore an object of the present invention to provide RE.sub.1 Ba.sub.2 Cu.sub.4 O.sub.8 type and RE.sub.2 Ba.sub.4 Cu.sub.7 O.sub.1 type oxide superconductor material that is strongly-linked and that can significantly retain its critical current density in a magnetic field.
It is a further object of the present invention to provide RE.sub.1 Ba2Cu.sub.4 O.sub.8 type and RE.sub.2 Ba.sub.4 CU.sub.7 O.sub.x type oxide superconductor material having well aligned oxide superconductor grains.
It is yet a further object of the present invention to provide a method for producing strongly-linked oxide superconductor material on a large scale.